This invention relates to medical equipment and, more particularly, to integrated anesthesia machines where a patient is induced and retained under anesthesia and where various parameters of the patient and the anesthesia machine are continuously monitored.
Anesthesia machines and/or systems are regularly used to maintain a patient undergoing surgery in a state of anesthesia and include various components, including vaporizers, to produce an anesthetic vapor from a liquid anesthetic, flowmeters and control valves for introduction of a carrier gas to carry the anesthetic to the patient and a series of monitors that carefully monitor various conditions of the patient and the status and operating condition of the anesthesia system itself.
Current anesthesia machines carry a variety of such monitors, including CO2 monitors, volume monitors, oximeters, blood pressure monitors, airway pressure monitors and the like. Many of the monitors are integrated into the anesthesia machine in order to localize all such monitors into a convenient location and to make use of the function of each of the monitors. In addition, such machines also provide a variety of alarms that are activated to alert the user as to a condition that should be investigated so as to protect the patient who, for obvious reasons, is unable to alert the attending personnel as to any such problems.
The present invention relates specifically to the alarm known as an apnea alarm and which signals an alarm condition when there is an indication that breathing has ceased or has been reduced to an unacceptable level. At present, the apnea alarm is commonly triggered based upon signals from two monitors, the CO2 monitor and the volume monitor. The CO2 monitor analyzes the level of CO2 in the patient's breathing circuit for both inspired and expired gasses and that CO2 level produces a well known wave shape during normal ventilation. The volume monitor continuously monitors the volume of gas expired by the patient and its transducer or sensor is also located in the patient breathing circuit near the patient.
As indicated, both the CO2 monitor and the volume monitor can trigger an alarm to indicate an apnea condition. Typically, the apnea alarm is triggered if the volume monitor does not indicate an increase in exhaled volume from the patient within a specific period of time, for example 30 seconds. Similarly, the CO2 monitor must receive and monitor the representative waveform within, for example, the same period of time 30 seconds. If the well known, representative waveform is not present during that period, the apnea alarm is triggered by the CO2 monitor.
As seen, therefore, the apnea alarm can be triggered by the recognition of fault conditions by either or both of the volume alarm and the CO2 alarm thereby establishing somewhat redundant systems triggering that alarm.
There is, in such anesthesia systems, a problem with spurious alarms, that is alarms that are triggered and which alert the personnel to a apnea condition wherein fact the problem is not an apnea condition and the patient is not experiencing an apnea condition. Such spurious alarms are annoying to the attending personnel since their attention is diverted to investigating the alarm condition only to find out that no alarm condition actually existed. Eventually personnel could ignore such alarms or give such alarm less than full attention on the belief that a spurious alarm was being repeated. Obviously, such a situation is not desirable.
One typical spurious alarm occurs with the apnea alarm based upon the volume monitor. A reason for the spurious apnea alarms based upon the volume monitor is that its sensor is positioned in the exhalation line of the patient circuit where the exhalation is returning from the patient to the bellows of the anesthesia machine. Often leaks occur in the exhalation flow stream and, typically, uncuffed tracheal tubes leak around the outside of such tubes such that all of the exhalation does not pass through the volume monitor sensor. The volume alarm thus interprets the lack of a predetermined volume of exhalation as a fault condition and triggers the apnea alarm even though the patent is being breathed normally.
Accordingly, it would be advantageous to be able to selectively, at the users choice, disable the apnea alarm based upon the volume monitor to prevent such spurious alarms, however, obviously, one cannot do so where the overall alarm functions of the anesthesia system would be compromised.
In present anesthesia machines, it is not therefore possible to disable or turn off the apnea alarm based upon the volume monitor when the anesthesia system is utilizing mechanical ventilation, that is where the anesthesia ventilator is breathing for the patient. If, on the other hand, the anesthesia system is in the bagging mode, that is the mechanical ventilator is not being used but instead the doctor is manually bagging the patient, it is possible to disable the apnea alarm based upon the volume monitor since in the bagging mode, it is assumed that the physician is present at the side of the patient and is visually monitoring the patients condition. As such, the alarms are not as necessary as the physician will immediately detect the apnea condition in the patient and be able to take any corrective action needed.
Accordingly, therefore, the problem of spurious apnea alarms is limited to the time in which the anesthesia ventilator is in the mechanical ventilation mode and at that time, spurious alarms are possible based upon the volume monitor.